Electrophoretic insulating coating



;hol such as methanol. usually the preponderant constituent of :the

Patented Apr. 21, 1953 ELEGTROPHORETIC INSULATING COATING llenryT.1Swansnn,.Lancaster, Pa., assignor' to Radio corporation, of America,a corporation of. Delaware No Drawing. Application June 19, 1948, SerialNo. 34,156

4 Claims. (01. 204-181) The invention relatesto insulatingcoatingzmaterials and more particularly :to such materials containing.aluminum nitrate and the processof making the same.

The cataphoretic process of applying .an .insulating coating to a metalbase has heretofore beenused when the configuration of the base has madethe use of the dragor spray process aimpractical- For example, coiledheater filaments for indirectly heated cathodes are preferably coated bythe cataphoretic process since the dra or spray process might leaveinner surfaces of the filament inadequately covered.

Some coating materials employed in the past for application to a base bycataphoresis have included aluminum oxide, commercial aluminum nitrate,magnesium nitrate, water, ,andan alco- The aluminum oxide is solidsportion of the coating, the quantities of aluminum nitrate and magnesiumnitrate being variable and their relative amounts being determined bythe desired thickness of the coating. The nitrates provide the requirednegative radical oi the electrolyte, which includes .water and analcohol such .as methanol. The aluminum oxide, aluminum nitrate, andmagnesium nitrate acquire electrical charges when the solution issubjected to a potential difference at displaced portions thereof bysuitable electrodes, one of which may constitute the metal base to becoated. After the metal base is coated it is baked at a relatively hightemperature, for example, about 1600 C. todrive off the liquidconstituent thereof and to convert the magnesium nitrate and .aluminumnitrate to the oxides which possess good insulating properties.

One of the difficulties associated with prior procedures involving acataphoretic application of a coating of the kind described has beenthat the finished coating shows the presence of pits and a number ofradial cracks therein, which result in a peeling or chipping of thecoating from the base. There appeared to be no known way of preventingor-controlling this undesirable condition of the coating with the resultthat it has been difficult, if not impossible, to obtain a coatinghaving predictable characteristics.

Another difficulty resides in the lack of stability of the coatingmaterial, which heretofore has made it necessary that the coatingmaterial be used in a relatively short time after itspreparation.

An object of the invention is to provide an insulating coating materialof improved insulating and adherence characteristics.

It is another object to provide an insulating coating having arelativelysmooth surface, free from cracks.

. .A further object is to provide an improved. cataphoretic process forapplying an insulating coating toa base.

Another objectis .to provide a novel form of aluminum nitrate whichwill. be a contributing factor in obtaining, a relatively stablecoatingsolution .and,a.-c0nsequent uniformly smooth and adherent coatingmaterial.

,An additional object is to provide .a coating formulation withreproducible end results.

Still another object is to provide a coating which canbe depended uponfor close control of thickness of insulating material.

Further objects andfeatures will become clear .as the descriptioncontinues. .According to the invention an insulating. coating materialisprovided which may include the constituents before referred to, with theexception that the commercial aluminum nitrate is treated in .aspecialmanner so that its inclusion .inthe coating will render the coating freefrom chipping or peeling and. make the coating materialmore stable for asubstantially longer period of time than heretofore possible.

The treatment of the aluminum nitrate .referred to comprises subjectingcommercial ,aluminum nitrate which has the chemical desi nation ofAI(,NO3).3.9H2O to a plurality of heat treatments in distilled waterwherein theheat is kept below the temperatureat which the aluminumnitrate will yield aluminum oxide. The yield of thistreatment isaluminumnitrate and aluminum hydroxide which has a non-crystalline structure.The heat treatments referred to are repeated until the yield has analuminumoxide equivalent of from 20% to 33%. These heat treatmentsconstitute a process by which the.for mula for commercial aluminumnitrate is changed. to a value such that the resultant material ischemically equivalent to from 20 to 33% by weight of aluminumoxide.

This relationship between aluminum oxide and aluminum nitrate can bestbe shown by the following theoretical consideration.

.A1(NO3),3.9H2O is. equivalent to, or for present purposes ispotentially capable of producing, aluminum oxide (A1203) to an extent ofabout 13.6% by weight. Aluminum nitrate-A1(N0a)a without any water ofcrystallization is chemically equivalent to about 24% A1203 by weight.Aluminum hydroxide (Al(OH) 3) is chemically equivalent to;65.4'%aluminum oxide (A1203) byweight.

Thus the formulation mostsuitable for providing between 20 and 33%aluminum oxide equivalent may be anhydrous aluminum nitrate with somefurther decomposition of part of this material to aluminum hydroxide orsome similar compound plus nitric acid which later is eliminated by theprocessing.

The decomposition would take place in accordance with the followingreaction:

Al(NOa)a 311,0 ARCH); 3HNO3T 1) 2Al(NOa) Heat A110; 3NzO5T 2) Heatingcommercial aluminum nitrate (A1(NO3)3.9H20) as outlined in the aboveprocesses gives rise to the following reactions, which take placesimultaneously: I

A1(NO3) 31130 Heat Al(OH) a SHNOaT (4) Thus the processing primarilyinvolves a reduction of water and nitric acid content of the aluminumnitrate and the formation of some aluminum hydroxide which tend tostabilize the solution and supply the proper conditions for properdeposition of the insulating material.

It can be seen from the equations shown above that it is possible toeliminate only the water of crystallization from commercial aluminumnitrate Al(NO3)3-9H20- and reach a formula corresponding to an aluminumoxide content within the range specified. Thus a single heat treatmentas described above would be sufficient to produce the formulationprescribed. This is but a step in the right direction, however, sincesingle evaporation has not been sufficient in practice for best results.The subsequent heat treatments serve to produce aluminum hydroxide andnitric acid, the latter being eliminated according to the reactionindicated in Equation 4. The extent of the reaction required forpractising the invention may be determined by actual plating tests.Although my experience has been that more than one evaporation should beemployed, it is possible that by proper control of the rate ofevaporation one treatment would suffice.

It can be seen readily that a number of theoretical combinations ofAl(NO3)3, H20, and Al(OI-I)3 could be obtained with an equivalent offrom 13.6% to 65.4% A1203. One thing is certain, however, as can bereadily seen from the above relationships, in order to produce amaterial which is equivalent to more than 24% A1203 it is necessary tohave some decomposition of Al(Na)3 to Al(0H)3 and HNO3 (nitric acid)which latter is driven oil by the heat treatment. According to theinvention, the primary requisite is that the A1203 equivalent be greaterthan 20%. Best results are obtained with materials potentially capableof producing or chemically equivalent to from 20% to 33% A1203. It ispossible, however, and theoretically probable, that material equivalentto more than 33% aluminum oxide (A1203) could be used satisfactorily.

Aluminum oxide (A1203) is used as a means of comparison mainly becausethe simplest analytical procedure for control of these materialsinvolves converting them to this product in accordance with thefollowing reactions:

These equations represent standard methods of analysis. The heatrequired in these cases is that of the Bunsen burner. Arrows pointingupward indicate that the compound so marked is driven off by the heat.

By way-of example, the heat treatment of the invention may be carriedout by placing about 0.5 lb. commercial aluminum nitrate in a largeevaporating dish and heating for about thirty-five minutes at atemperature below 135 C. but allowing the material to approach thistemperature as the evaporation proceeds. The material initially becomesliquid at the end of fifteen minutes but reverts to its solidnon-crystalline state after an additional twenty minutes have elapsed.In this state the material may be said to have a syrupy consistency.During the heating step constant stirring of the mixture is necessaryand the frequency of the stirring should be increased when the materialbegins to solidify.

To the solid material is then added from 300 to 400 cc. of distilledwater, in which the material is caused to dissolve by a gentle heatingthereof. The temperature is then raised to a value below 135 C. untilthe water has evaporated and the material has returned to the solid andsomewhat syrupy state. This step is repeated once more, after which thematerial is subjected to a test to determine its aluminum oxideequivalent. If the aluminum oxide equivalent is less than 20% of thetreated aluminum nitrate, the step referred to must be repeated untilthis value of aluminum oxide equivalent, or a value not exceeding 33%,is secured. It is then submitted to a practical test designed tofunction as a control of suitability for use.

The test employed to determine the suitability of the processed aluminumnitrate for the sake of convenience is one of actual application in acoating suspension. It consists of preparation of asmall quantity ofcoating solution and application of the insulating material in a regularfashion, If the result is a smooth coating free from cracks before andafter firing, the prepared nitrate is approved for use. If the coatingis rough or cracked further treatment of the nitrate is necessary.

The invention produces a new material adapted to serve as a constituentof an insulating coating and having the desirable properties ofimparting to the coating material a stability of conductivity resultingfrom a desired value of concentration of hydrogen ions at a desirablelevel per unit of volume. The reduction in the amount fo water ofcrystallization of aluminum nitrate from 9 molecules per molecule ofaluminum nitrate to a lower value accomplished by the invention,eliminates a source of water having a tendency to hydrolyze to nitricacid during storage of the material. Nitric acid reduces the stabilityof the coating material and its elimination, or at least substantialreduction, results in a relatively stable solution.

A further feature of the invention concerns the relative amounts ofmagnesium nitrate and the treated aluminum nitrate heretofore described,required in an insulating coating for best results. For example, acoating material according to the invention may include 2358.7 gramsAlundum, 1620 cc. of a solvent such as denatured alcohol, 1500 cc. ofdistilled water 47.25 grams of magnesium nitrate, and an amount ofaluminum nitrate determined by the following formula:

expressed as a fraction of the aluminum nitrate which should lie betweenthe extremes of .20 to Weight of aluminum nitrate= .33. The numerator13.95 is a constant and I represents the aluminum oxide equivalent ofaluminum nitrate in grams to be used with 47.25 grams of magnesiumnitrate.

Various modifications may be made in the invention without departingfrom its spirit and it is intended to include them Within the scope ofthe appended claims.

I claim:

1. Method of preparing aluminum nitrate for use in a relatively stablesolution for application as an insulating coating comprising heatingcommercial aluminum nitrate to drive off at least part of the water ofcrystallization content thereof and to convert a portion of saidaluminum nitrate to aluminum hydroxide, and until the treated aluminumnitrate and the derived aluminum hydroxide contain an aluminum oxideequivalent of from 20 to 33% by weight.

2. An electrophoretic insulating coating material comprising aluminumoxide, a solvent, magnesium nitrate and aluminum nitrate having analuminum oxide equivalent of from 20 to 33% by weight, the amount ofsaid aluminum nitrate being related to the amount of said magnesiumnitrate and the aluminum oxide equivalent of said aluminum nitrate, theamount of said aluminum oxide equivalent being from .20 to .33 of saidaluminum nitrate, the amount of said aluminum nitrate being such as toyield 13.95 grams of aluminum oxide, and the amount of said magnesiumnitrate being 47.25 grams.

3. Method of preparing a relatively stable insulating coating materialcontaining aluminum nitrate, comprising heat treating commercialhydrolyzed aluminum nitrate to increase its aluminum oxide equivalent,mixing a relatively small portion of said coating material contain ingsaid heat treated aluminum nitrate, plating said coating material on ametal base as a test to determine the amount of said aluminum oxideequivalent, and repeating said heat treatment and said test until saidtest produces a relatively smooth coating, said relatively smoothcoating indicating an aluminum oxide equivalent of from 20 to 33% byweight of the treated aluminum nitrate.

4. An electrophoretic insulating coating material comprising aluminumoxide, magnesium nitrate, aluminum nitrate and a solvent, the amount ofsaid aluminum nitrate being determined by dividing a constant having avalue of 13.95 by the aluminum oxide equivalent of said aluminumnitrate, said aluminum oxide equivalent having a range of from 20 to 33%of said aluminum nitrate by weight.

HENRY '1. SWANSON.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 249,275 Varney Nov. 8, 1881 1,792,410 Buchner Feb. 10, 19311,849,082 Eyer Mar. 15, 1932 1,958,710 Moyer May 15, 1934 2,034,599 VanMarle Mar. 17, 1936 2,036,508 Svendsen Apr. 7, 1936 2,127,504 Derr etal. Aug. 3, 1938 2,307,018 Cardell Jan. 5, 1943 2,376,047 George et a1.May 15, 1945 OTHER REFERENCES Roscoe et al., Treatise on Chemistry, vol.II (1907), page 726.

2. AN ELECTROPHORETIC INSULATING COATING MATERIAL COMPRISING ALUMINUMOXIDE, A SOLVENT, MAGNESIUM NITRATE ANS ALUMINUM NITRATE HAVING ANALUMINUM OXIDE EQUIVALENT OF FROM 20 TO 33% BY WEIGHT, THE AMOUNT OFSAID ALUMINUM NITRATE BEING RELATED TO THE AMOUNT OF SAID MAGNESIUMNITRATE AND THE ALUMINUM OXIDE EQUIVALENT OF SAID ALUMINUM NITRATE, THEAMOUNT OF SAID ALUMINUM OXIDE EQUIVALENT BEING FROM .20 TO .33 OF SAIDALUMINUM NITRATE, THE AMOUNT OF SAID ALUMINUM NITRATE BEING SUCH AS TOYIELD 13.95 GRAMS OF ALUMINUM OXIDE, AND THE AMOUNT OF SAID MAGNESIUMNITRATE BEING 47.25 GRAMS.